This invention relates to an improvement of an electric discharge machining method.
In general, in an electric discharge machining operation, a short-circuit occurs between the wire electrode and the workpiece with considerably high probability for instance because the discharge machining traces often inflate, or waste materials, such as sludge, are formed during machining.
If a short-circuit occurs in the region of the workpiece where high pulse current is used as in the case of roughing machining, or high frequency machining pulse voltage is employed, then the average current is increased, and therefore the Joule heat thereof may damage the electrode and the workpiece, or the average current thus increased may damage the transistors in the electric discharge machine.
In order to overcome this difficulty, Japanese Patent Application (OPI) No. 500/1975 (the term "OPI" as used herein means an "unexamined published application") for instance has been disclosed in the art.
In the prior art, the conditions of the gap between the electrode and the workpiece are detected so that a certain number of voltage pulses are eliminated from the train: of voltage pulses according to the conditions of the gap thus detected.
One example of a conventional electric discharge machine of this type is as shown in FIG. 21. In FIG. 21, reference numeral 1 designates a DC power source; 2, a switching transistor; 3, an emitter resistor; 4, a tool electrode; 5, a workpiece; 6, a short-circuit detecting device; 7, a discharging pulse thinning signal generating device; and 8, a control device for selectively applying voltage to the base of the switching transistor 2 to render the latter on and off. In a stable electric discharge machining operation, the transistor 2 is rendered conductive in response to a control signal from the control device 8, to apply discharging current through the transistor 2 and the emitter resistor 3 to the gap between the electrode 4 and the workpiece 5 to machine the latter 5.
In this case, the electric discharge machining operation advances as shown in FIG. 22. Electric discharge occurs first where, as shown in FIG. 22(a), is made most suitable for electric discharge for instance because of an uneven part of the workpiece' surface or decomposition of materials contained in the machining solution. Next, electric discharge occurs between the electrode and the workpiece in such a manner that it is spread therein. In this case, the workpiece will be stably machined as shown in FIG. 22(c).
When, on the other hand, waste materials and other decomposed materials are deposited at a position as shown in FIG. 22(d), then electric discharge is caused concentrating at one position. As a result, a electric discharge trace is markedly formed at the position resulting in an occurrence of defects in the machined surface.
If a short-circuit occurs between the electrode 4 and the workpiece 5 because of waste materials for instance, then it is detected by the short-circuit detecting device 6, which applies a detection signal to the discharge pulse thinning signal generating device 7. Thereupon, the device 7 supplies the discharge pulse thinning signal, which is applied to the control device 8. As a result, rendering the transistor conductive is suspended for a period of time corresponding to a predetermined number of voltage pulses from the next one. That is, when a short-circuit occurs between the electrode and the workpiece, the predetermined number of voltage pulses are removed from the train of voltage pulses, whereby the concentration of electric discharge to one position is eliminated, and accordingly failures in electric discharge machining can be prevented, such as damaging workpieces.
An electric discharge machine generally employs a method of maintaining the voltage of the machining gap substantially constant, thereby maintaining the machining gap constant. In the method, in the case where the machining gap is high in the degree of insulation for instance because the machining solution in the machining gap is clean, the machining gap is controlled to be narrow; whereas in the case where the degree of insulation is low for instance because a number of waste materials are accumulated in the machining gap, the machining gap is controlled to be wide.
For instance at the start of an electric discharge machining operation, a clean machining solution is in the machining gap, and therefore the machining gap is considerably high in the degree of insulation, and it is therefore controlled to be narrow. Accordingly, during the initial period of an electric discharge machining operation, a short-circuit is liable to occur, with high percentage, at the machining gap by the inflation of a discharge machining trace. Therefore, application of high frequency voltage pulses, in this case, will cause electric discharge frequently.
In the initial period of a electric discharge machining operation, the removal of voltage pulses in response to the occurrence of a short-circuit in the machining gap will reduce the probability of failure in discharge machining. However, the method suffers from a difficulty that, when the machining operation becomes normal by the removal of voltage pulses after the occurrence of the short-circuit, the frequency of the voltage pulses is increased again, thus causing short-circuits again. This is repeatedly carried out as shown in FIG. 23.
In FIG. 23, reference character B designates a train of voltage pulses for discharge machining. Upon occurrence of a short-circuit, a predetermined number of voltage pulses D are removed from the train of voltage pulses B. That is, the conventional method in which, when a short-circuit occurs in the machining gap because of the unsatisfactory conditions thereof, a certain number of voltage pulses are removed, and when the machining conditions become stable, the machining operation is normally carried out again, suffers from a difficulty that, if the machining condition is on the border line between stable condition and unstable condition, it is difficult to change the machining condition into its original stable condition, and the machining efficiency is lowered by repeatedly removing the certain number of voltage pulses as shown in FIG. 23.
When, at the start of an electric discharge machining operation, a short-circuit occurs in the above-described manner, then the application of current is suspended, and the electrode is mechanically moved away from the workpiece to eliminate the contact therebetween which is caused by the inflation of the discharge machining trace for instance. Thereafter, the electrode is set close to the workpiece again. Hence, in the conventional method, it is difficulty to quickly restore the machining condition, thus, the machining efficiency is accordingly low.